Heterogeneity and territorial organization of the nuclear matrix and related structures

Cardiac hormones for the treatment of cancer

Four cardiac hormones, namely atrial natriuretic peptide, vessel dilator, kaliuretic peptide, and long-acting natriuretic peptide, reduce up to 97% of all cancer cells in vitro. These four cardiac hormones eliminate up to 86% of human small-cell lung carcinomas, two-thirds of human breast cancers, and up to 80% of human pancreatic adenocarcinomas growing in athymic mice. Their anticancer mechanisms of action, after binding to specific receptors on cancer cells, include targeting the rat sarcoma-bound GTP (RAS) (95% inhibition)-mitogen-activated protein kinase kinase 1/2 (MEK 1/2) (98% inhibition)-extracellular signal-related kinase 1/2 (ERK 1/2) (96% inhibition) cascade in cancer cells. They also inhibit MAPK9, i.e. c-Jun N-terminal kinase 2. They are dual inhibitors of vascular endothelial growth factor (VEGF) and its VEGFR2 receptor (up to 89%). One of the downstream targets of VEGF is β-catenin, which they reduce up to 88%. The WNT pathway is inhibited up to 68% and secreted frizzled-related protein 3 decreased up to 84% by the four cardiac hormones. AKT, a serine/threonine protein kinase, is reduced up to 64% by the cardiac hormones. STAT3, a final 'switch' that activates gene expression that leads to malignancy, is decreased by up to 88% by the cardiac hormones. STAT3 is specifically decreased as they do not affect STAT1. There is a cross-talk between the RAS-MEK 1/2-ERK 1/2 kinase cascade, VEGF, β-catenin, WNT, JNK, and STAT pathways and each of these pathways is inhibited by the cardiac hormones.

Nuclear matrix isolated from plant cells

Residual nuclear matrices can be successfully obtained from isolated nuclei of different monocot and dicot plant species using either high ionic or low ionic extraction protocols. The protein composition of isolated nuclear matrices depends on the details of isolation protocols. They are stable and present in all cases, a tripartite organization with a lamina, nucleolar matrix, and internal matrix network, and also maintain some of the basic architectural features of intact nuclei. In situ preparations demonstrate the continuity between the nuclear matrix and the plant cytoskeleton. Two-dimensional separation of isolated plant nuclear matrix proteins reveals a heterogeneous polypeptide composition corresponding rather to a complex multicomponent matrix than to a simple nucleoskeletal structure. Immunological identification of some plant nuclear matrix components such as A and B type lamins, topoisomerase II, and some components of the transcription and splicing machineries, internal intermediate filament proteins, and also specific nucleolar proteins like fibrillarin and nucleolin, which associate to specific matrix domains, establish a model of organization for the plant nuclear matrix similar to that of other eukaryotes. Components of the transcription, processing, and DNA-anchoring complexes are associated with a very stable nucleoskeleton. The plant matrix-attached regions share structural and functional characteristics with those of insects, vertebrates, and yeast, and some of them are active in animal cells. In conclusion, the available data support the view that the plant nuclear matrix is basically similar in animal and plant systems, and has been evolutionarily conserved in eukaryotes.

Heterochromatin protein 1 is required for correct chromosome segregation in Drosophila embryos

Heterochromatin protein 1 is associated with centromeric heterochromatin in Drosophila, mice, and humans. Loss of function mutations in the gene encoding heterochromatin protein 1 in Drosophila, Suppressor of variegation2-5, decrease the mosaic repression observed for euchromatic genes that have been juxtaposed to centromeric heterochromatin. These heterochromatin protein 1 mutations not only suppress this position-effect variegation, but also cause recessive embryonic lethality. In this study, we analyze the latter phenotype in the hope of gaining insight into heterochromatin function. In our analyses of four alleles of Suppressor of variegation2-5, the lethality was found to be associated with defects in chromosome morphology and segregation. While some of these defects are seen throughout embryonic development, both the frequency and severity of the defects are greatest between cycles 10 and 14 when zygotic transcription of the Suppressor of variegation2-5 gene apparently begins. By this time in development, heterochromatin protein 1 levels are diminished by four-fold in a quarter of the embryos produced by parents that are both heterozygous for a null allele (Suppressor of variegation2-5(05)). In a live analysis of the phenotype, we find prophase to be lengthened by more than two-fold in Suppressor of variegation2-5(05) mutant embryos with subsequent defects in chromosome segregation. The elongated prophase suggests that the segregation phenotype is a consequence of defects in events that occur during prophase, either in chromosome condensation or kinetochore assembly or function. Immunostaining with an antibody against a centromerespecific antigen indicates that the kinetochores of most chromosomes are functional. The immunostaining results are more consistent with defects in chromosome condensation being responsible for the segregation phenotype.

Heterochromatin protein 1 distribution during development and during the cell cycle in Drosophila embryos

Heterochromatin protein 1 (HP1) was initially discovered as a protein that is associated with the heterochromatin at the chromocenter of polytene chromosomes in Drosophila larval salivary glands. In this paper we investigate the localization of heterochromatin protein 1 in the diploid nuclei of Drosophila embryos. We focus on its association with the interphase heterochromatin in fixed embryos before and during cycle 14, the developmental time at which heterochromatin becomes most conspicuous, and also follow its localization during mitosis. The GAGA transcription factor was recently shown to be localized at sequences within alpha-heterochromatin in pre-cycle 14 embryos, and an antibody against this protein serves as a convenient marker for these sequences. We find an enrichment of heterochromatin protein 1 in the intensely DAPI-staining regions near the apical surface of nuclear cycle 10 embryos. At this stage GAGA factor is localized into punctate structures in this same region. This enrichment for HP1 is markedly increased during nuclear cycle 14. Surprisingly, whereas GAGA factor retains its association with the heterochromatin throughout the cell cycle, a significant fraction of HP1 is dispersed throughout the spindle around the segregating chromosomes during mitosis. This dispersed pool of heterochromatin protein 1 was observed during mitosis in both early and late Drosophila embryos and in an analysis of a bacterially produced 6x histidine-heterochromatin protein 1 fusion protein injected into living Drosophila embryos. When Drosophila tissue culture cells were prepared by a method which removes soluble protein and avoids fixation of the mitotic chromosomes, an enrichment for heterochromatin protein 1 in the heterochromatin of the chromosomes was discovered also.

Interphase chromosomes of Friend-S cells are attached to the matrix structures through the centromeric/telomeric regions

DNA of the attachment sites of Friend erythroleukemia cells, isolated according to the conventional procedure, represents short, nuclease-resistant fragments with sizes below 400 bp, belonging to the class of mouse satellite. A number of experiments have indicated that their unusual resistance is due to complexing with RNA. By various approaches, it was confirmed that similar fragments might be recovered from total DNA following extensive digestion with DNase I. In situ hybridizations revealed further that at mitosis the sequences of the attachment sites are located at the centromeric/telomeric regions of the chromosomes, while at interphase they are redistributed into 9-13 well-defined clusters spread throughout the entire nuclear area. Parallel biochemical and electronmicroscopic studies have clarified, moreover, that the all three compartments of the matrix harbor such sequences. Thus, it appears that the attachment sites described function only at interphase, anchoring the both ends of each interphase chromosome to the matrix structures.

Ultrastructural morphometry and the control of lymphocyte nuclear size in non-Hodgkin lymphoma: a review

Morphologic features, particularly nuclear characteristics, continue to be a major factor in the classification of the subtypes of non-Hodgkin lymphoma (NHL). Information is limited or nonexistent, however, concerning the mechanisms responsible for nuclear appearance. This is particularly true for nuclear size. In determining which of the nuclear compartments--condensed chromatin or the interchromatinic (euchromatin or nuclear matrix) region--is responsible for nuclear size, ultrastructural morphometric analysis has been a useful tool. Linear regression analysis has been used to compare, in turn, condensed chromatin area (CCA) and interchromatinic area (ICA) against nuclear area (NA). Whether in normal or neoplastic lymphocytes, correlation coefficients are moderately good when CCA is matched against NA, but CCA is only a weak predictor of NA. In contrast, correlation coefficients are remarkably high for ICA compared to NA, and ICA predicts NA to the degree of 94% to 97%. This holds true for all NHLs tested and for reactive hyperplasias. Despite abnormalities in lymphocyte nuclear size in NHL, the size of any nucleus is controlled by the amount of interchromatinic material to an exceptional degree. Even so, the mechanisms responsible for major changes in nuclear volume during transformation of normal lymphocytes are apparently altered in NHL. Morphometric analysis, ultrastructurally, can assist in understanding, testing, and defining diagnostic morphologic criteria in this disease.

Nuclear matrins: identification of the major nuclear matrix proteins

A preparative two-dimensional polyacrylamide gel system was used to separate and purify the major Coomassie blue-stained proteins from the isolated rat liver nuclear matrix. Approximately 12 major proteins were consistently found. Of these, 5 proteins represented identified proteins, including nuclear lamins A, B, and C, the nucleolar protein B-23, and residual components of core heterogeneous nuclear ribonucleoproteins. The remaining eight major proteins termed the nuclear matrins consisted of matrin 3 (125 kDa, slightly acidic), matrin 4 (105 kDa, basic), matrins D-G (60-75 kDa, basic), and matrins 12 and 13 (42-48 kDa, acidic). Peptide mapping and two-dimensional immunoblot studies indicate that matrins D-G compose two pairs of related proteins (matrins D/E and F/G) and that none of the matrins resemble the nuclear lamins or any of the other major proteins detected on our two-dimensional gels. Subfractionation immunoblot experiments demonstrated the nearly exclusive localization of matrins F/G and other matrins to the nuclear matrix fraction of the cell. These results were further supported by indirect immunofluorescence microscopy that showed a strictly interior nuclear localization of the matrins in intact cells in contrast to the peripherally located nuclear lamins. We conclude that the nuclear matrins are a major class of proteins of the nuclear matrix interior and are distinct from the nuclear lamins.

Association of topoisomerase II with the hepatoma cell nuclear matrix: the role of intermolecular disulfide bond formation

Previous studies have resulted in conflicting data regarding the recovery of the nuclear enzymes topoisomerase (topo) II and topo I in the nuclear matrix fraction. In the present study we have assessed the effect of systematically altering a single extraction procedure on the distribution of these enzymes during the subfractionation of nuclei from HTC hepatoma tissue culture cells. When nuclear monolayers (prepared by treating attached cells in situ with the neutral detergent Nonidet-P40 at 4 degrees C) were isolated in the presence of the irreversible sulfhydryl blocking reagent iodoacetamide, subsequent treatment with DNase I and RNase A followed by 1.6 M NaCl resulted in structures which were extensively depleted of intranuclear components as assessed by phase contrast microscopy and conventional transmission electron microscopy. These structures contained 12 +/- 4% of the total protein present in the original nuclear monolayers. The lamins and polypeptides with molecular weights comparable to those of actin and vimentin were the predominant polypeptides present on SDS-polyacrylamide gels. Western blotting revealed that less than 5% of the total nuclear topo II molecules were present in these structures. In contrast, when the sulfhydryl cross-linking reagent sodium tetrathionate (NaTT) was substituted for iodoacetamide, the same extraction procedure yielded structures containing components of the nucleolus and an extensive intranuclear network. These structures contained a wide variety of nonlamin, nonhistone nuclear polypeptides including 23 +/- 4% of the total nuclear topo II. SDS-polyacrylamide gel electrophoresis performed under nonreducing conditions revealed that topo II in these nuclear matrices was present as part of a large disulfide cross-linked complex. Treatment of these structures with reducing agents in 1.6 M NaCl released the topo II. In contrast, topo I did not form disulfide cross-linked oligomers and was not detectable in any of these nuclease- and salt-resistant structures prepared at 4 degrees C. To assess the effect of in vitro heat treatment on the distribution of the topoisomerases, nuclear monolayers (isolated in the absence of iodoacetamide and NaTT) were heated to 37 degrees C for 1 h prior to treatment with nucleases and 1.6 M NaCl. The resulting structures (which retained 26 +/- 5% of the total nuclear protein) were morphologically similar to the NaTT-stabilized nuclear matrices and contained 15 +/- 4% of the total nuclear topo II. High-molecular-weight disulfide cross-linked oligomers of topo II were again demonstrated. Attempts to demonstrate these disulfide cross-linked oligomers in intact cells were unsuccessful.

Interphase nuclear envelope lamins form a discontinuous network that interacts with only a fraction of the chromatin in the nuclear periphery

Antibodies directed against nuclear envelope lamin proteins have been used in conjunction with three-dimensional light and electron microscope methodologies to determine the spatial organization of lamins in diploid interphase nuclei and to relate this organization to the positions of chromatin in the nuclear periphery. Using Drosophila early embryos, Drosophila Kc cells, and human HeLa cells, it is qualitatively and quantitatively observed that lamins are organized as a highly discontinuous, apparently fibrillar network that leaves large voids in the nuclear periphery containing little or no lamin. Using fluorescence microscopy to compare and quantitate the relationship between chromatin and the lamin network, it is found that although there is a strong tendency for the most peripheral chromatin to be positioned directly underneath a lamin fiber, only a small fraction of the chromatin in the nuclear periphery is sufficiently close to a lamin fiber to possibly be in direct contact.

Low-abundance 32-kilodalton nuclear protein specifically enriched in the central nervous system

Recently, a low-abundance nuclear protein, p32/6.3, has been identified in brain tissue (Egle and Shelton: Journal of Biological Chemistry 261:2294-2298, 1986). Using a Western blot procedure, we describe its distribution in the nervous system, determine its relative enrichment in brain versus liver, kidney, and certain other tissues, and describe an isolation procedure from brain. Selective enrichment occurs in basal ganglia, diencephalon, hippocampus, cerebellum, brainstem, spinal cord, and cerebral cortex but not in retina, dorsal root ganglia, and sympathetic ganglia. Thus, enrichment is limited to areas of the central nervous system. p32/6.3 appears to be preferentially enriched in neurons, because in bulk-isolated fractions from rat grey matter it is more abundant in neuron-enriched fractions than in astrocyte-enriched fractions. p32/6.3 is approximately 20-fold more concentrated in an insoluble nuclear protein or matrix fraction from forebrain than from kidney, liver, adrenal gland, or retina. This degree of enrichment is an ancient trait, detectable in the chicken as well as mammals.

Identification of La ribonucleoproteins as a component of interchromatin granules

Monoclonal antibodies raised against the La antigen were used to localize by preembedding immunoelectron microscopy, snRNPs containing this protein. The results demonstrate that La RNPs are localized in clusters of interchromatin granules, both in Triton X-100-extracted and DNase-digested nuclei. DNase-digested salt-extracted nuclei contained, in addition, labeled structures identified as perichromatin granules and fibers. A close association of labeled granules with the nucleoli was also observed. Digestion of nuclei with DNase yielded residual scaffolds of intermediate filaments and nuclear lamina devoid of interchromatin granules and La immunostaining. Release of the La antigen was tested in the presence of ATP and cytochalasin B. Only cytochalasin was effective suggesting a role for nuclear actin in anchorage of snRNPs.

Autonomous splicing and complementation of in vivo-assembled spliceosomes

We have used an in vivo system generating assayable amounts of a specific pre-mRNA to study the relationship between splicing and an operationally defined nuclear matrix preparation (NM). When NM is prepared by extraction of DNase I-treated nuclei with an approximately physiological concentration of KCl (0.1 M), a portion of NM-associated precursor can be spliced in vitro in the presence of ATP and Mg2+ and in the absence of splicing extract ("autonomous splicing"). We propose that the autonomous reaction, which does not exhibit a temporal lag and is half-complete in 5 min, occurs in fully assembled, matrix-bound ribonucleoprotein complexes (in vivo spliceosomes). Extraction of the NM with concentrations of KCl greater than 0.4 M eliminates autonomous splicing but leaves behind preassembled complexes that can be complemented for splicing with HeLa cell nuclear extract. The splicing complementing factor, representing one or more activities present in the nuclear extract and also in the cytoplasmic S100 fraction, is relatively heat resistant, devoid of an RNA component, and does not bind to DEAE-Sepharose in 0.1 M KCl. It exists in the nucleus in two forms; bound to autonomous spliceosomes and free in the nucleoplasm. Biochemical features of the complementation reaction, and conditions for reversible uncoupling of the two splicing steps are described and discussed.

Centromeric association and non-random distribution of centromeres in human tumour cells

Centromere arrangement in interphase and metaphase cells of two human tumour cell lines was analysed using anti-kinetochore antibodies as immunofluorescent probes. In GLC1 interphase nuclei, kinetochores were non-randomly positioned around the nucleolus and close to the nuclear membrane. During S and early G2 phase, necklace-like strands of kinetochores were formed in the centre of the nucleus. The duplication of sister kinetochores during the G2 phase was not synchronized. At late G2 phase, a relatively random topological distribution of centromeres was observed with short linear arrays of sister kinetochores. Carefully spread metaphase plates of MDA-MB231 cells generally exhibited a linear alignment of centromeres and large centromeric clusters. In completely pulverized MDA-MB231 cells, centromeres showed a strong tendency to associate with each other.

Cell cycle dependent distribution of the proliferation-associated Ki-67 antigen in human embryonic lung cells

The cell cycle-dependent distribution of the proliferation-associated Ki-67 antigen has been evaluated immunocytochemically in L-132 human fetal lung cells. The cells were synchronized and cell cycle phases were determined: G1 = 6.7 h, S = 5.4 h, G2 = 8.5 h and mitosis = 1.3 h. The Ki-67 patterns were strictly correlated with the cell cycle phases. In late G1-phase, Ki-67 antigen was present only in the perinucleolar region. In the S-phase, Ki-67 staining was found homogeneously in the karyoplasm and in the perinucleolar region. G2-phase cells contained a finely granular Ki-67 staining in the karyoplasm with Ki-67-positive specks and perinucleolar staining. In early mitotic cells (pro- and metaphase) an intense perichromosomal Ki-67 staining was observed in addition to a homogeneously stained karyoplasm in prophase, and cytoplasm in metaphase. During ana- and telophase the Ki-67 antigen disappeared rapidly. In resting cells there was no Ki-67 staining.

Intranuclear distribution of SV40 large T-antigen and transformation-related protein p53 in abortively infected cells

The intranuclear localization of SV40 T-antigen (T-Ag) and the cellular protein p53 was studied in SV40 abortively infected baby mouse kidney cells using two complementary methods of ultrastructural immunocytochemistry in combination with preferential staining of nuclear RNP components and electron microscope autoradiography. Both proteins were revealed in association with peri- and interchromatin RNP fibrils containing the newly synthesized hnRNA. In addition, T-Ag and p53 remained bound, at least in part, to the residual internal nuclear matrix following nuclease and salt extractions of infected cells. The localization of T-Ag was different in SV40 lytically infected monkey kidney cells since, in addition to hnRNP fibrils, the viral protein was also associated with cellular chromatin. However, when lytic infection was performed in conditions of blocked viral DNA replication, T-Ag was no longer associated with the cellular chromatin but remained bound to the hnRNP fibrils. We conclude that the transforming and lytic functions of T-Ag can be distinguished by different subnuclear distributions. The significance of the association of T-Ag and p53 with hnRNP fibrils and the internal nuclear matrix is discussed in relation to the role of these structures in the control of cellular mRNA biogenesis.

Lamin disassembly kinetics: a cell-free system with extracts from mitotic HeLa cells

We describe a cell-free system in which a postribosomal supernatant from metaphase HeLa cells induces prophase-like changes in permeabilized HeLa cell populations as evidenced by the nuclear lamin disassembly and chromatin condensation. We have attempted to characterize the cell-free system with permeabilized HeLa cells. First, by extracting lamins with agents known to disrupt the noncovalent interactions in the supramolecular lamin aggregate in interphase using polyclonal and a newly established monoclonal anti-lamin Ab 2E3, uniform extraction of lamins was achieved with urea and deoxycholate whereas the cation Mg2+ and 2-mercaptoethanol had little effect on the disassembly of interphase lamins. Second, cytoplasmic extract from mitotic HeLa cells, synchronized by a nitrous oxide metaphase arrest, was tested. It had a differential effect on interphase lamin depolymerization. Nuclei in G1 phase of the cell cycle were more resistant against the mitotic extracts than cells in S and G2 phase. The results are discussed in terms of a possible inactivation of mitotic extracts by factors present in nuclei in early interphase.

Changes in transcriptional activity and matrix association of alpha 2u-globulin gene family in the rat liver during maturation and aging

Hepatic synthesis of alpha 2u-globulin in the male rat begins at puberty (about 40 days), reaches a peak level at about 80 days, and ceases at about 750-800 days of age. The age-dependent changes in alpha 2u-globulin synthesis are correlated with both the steady-state level of the hepatic mRNA for this protein and the rate of transcription of the alpha 2u-globulin gene family. Transcriptional activation of the alpha 2u-globulin gene family at puberty and cessation of transcription at senescence correlate with the association and dissociation of this gene domain with the nuclear matrix. Unlike the alpha 2u-globulin gene, the albumin gene in the liver shows preferential association with the nuclear matrix throughout the life. From these results we conclude that the age-dependent changes in alpha 2u-globulin synthesis are due to the alteration in the rate of transcription of the alpha 2u-globulin gene, and that the association of this gene domain to the nuclear matrix is a prerequisite to its transcriptional activation.

Chromosomes and their relationship to nuclear components during the cell cycle in Chinese hamster cells

Chromosomes and their relationship to nuclear components during various phases of the cell cycle were studied with different fixation, embedding, and enzyme techniques. The results showed that interphase chromosomes may have oriented in such a way that a given locus became associated with the nuclear membrane. Some chromosomes also appeared to interact with the nucleolus. The nuclear matrix materials, however, were distributed between the chromosomes and formed a delineating boundary for the chromosomes. These matrix materials, furthermore, formed channel-like structures within the nucleus and towards the cytoplasm through their interaction with nuclear pore complexes. During mitosis, chromosomes were encapsulated with material that appeared to be derived from the matrix, disintegrated residues and fragments of the nuclear envelope, the lamina, and nucleolar material. These chromosome-associated materials seen in mitosis appeared to serve as foci for formation of new nuclear components in subsequent interphase.

Evidence for the existence of a nucleolar skeleton attached to the pore complex-lamina in human fibroblasts

This work deals with the types of nuclear skeletal structures obtained from human fibroblast nuclei isolated by different procedures. It is confirmed that, in somatic vertebrate cells, the pore complex-lamina is always observed, whereas the presence of internal nucleolar and extranucleolar residual structures depends upon the method of nuclear isolation used. Furthermore, the results reported here argue for the existence of a nucleolar skeleton different from the nucleolar matrix often observed in different cell types by other investigators. The conditions of nuclear isolation which allow us to visualize this nucleolar skeleton without any other internal residual structures are described. The attachment of the nucleolar skeleton to the lamina suggested by the present data is considered in relation to the in situ position of nucleoli near the nuclear envelope.

Pre-mRNA splicing and the nuclear matrix

We examined the relationship between pre-mRNA splicing and the nuclear matrix by using an in vivo system that we have developed. Plasmids containing the inducible herpesvirus tk gene promoter linked to an intron-containing segment of the rabbit beta-globin gene were transfected into HeLa cells, and then the promoter was transactivated by infection with a TK- virus. Northern analysis revealed that the globin pre-mRNA and all its splicing intermediates and products are associated with the nuclear matrix prepared from such transfected cells. When the nuclear matrix was incubated with a HeLa cell in vitro splicing extract in the presence of ATP, the amount of matrix-associated precursor progressively decreased without a temporal lag in the reaction, with a corresponding increase in free intron lariat. Thus, most of the events of the splicing process (endonucleolytic cuts and branching) occur in this in vitro complementation reaction. However, ligation of exons cannot be monitored in this system because of the abundance of preexisting mature mRNA. Since the matrix is not a self-splicing entity, whereas the in vitro splicing system cannot process efficiently deproteinized matrix RNA, we conclude from our in vitro complementation results (which can be reproduced by using micrococcal nuclease-treated splicing extract) that the nuclear matrix preparation retains parts of preassembled ribonucleoprotein complexes that have the potential to function when supplemented with soluble factors (presumably other than most of the small nuclear ribonucleoproteins known to participate in splicing) present in the HeLa cell extract.

Pre-mRNA splicing and the nuclear matrix

We examined the relationship between pre-mRNA splicing and the nuclear matrix by using an in vivo system that we have developed. Plasmids containing the inducible herpesvirus tk gene promoter linked to an intron-containing segment of the rabbit beta-globin gene were transfected into HeLa cells, and then the promoter was transactivated by infection with a TK- virus. Northern analysis revealed that the globin pre-mRNA and all its splicing intermediates and products are associated with the nuclear matrix prepared from such transfected cells. When the nuclear matrix was incubated with a HeLa cell in vitro splicing extract in the presence of ATP, the amount of matrix-associated precursor progressively decreased without a temporal lag in the reaction, with a corresponding increase in free intron lariat. Thus, most of the events of the splicing process (endonucleolytic cuts and branching) occur in this in vitro complementation reaction. However, ligation of exons cannot be monitored in this system because of the abundance of preexisting mature mRNA. Since the matrix is not a self-splicing entity, whereas the in vitro splicing system cannot process efficiently deproteinized matrix RNA, we conclude from our in vitro complementation results (which can be reproduced by using micrococcal nuclease-treated splicing extract) that the nuclear matrix preparation retains parts of preassembled ribonucleoprotein complexes that have the potential to function when supplemented with soluble factors (presumably other than most of the small nuclear ribonucleoproteins known to participate in splicing) present in the HeLa cell extract.

The nuclear skeleton and the spatial arrangement of chromosomes in the interphase nucleus of vertebrate somatic cells

The topologic distribution of interphase chromosomes established by using various cytologic methods and data concerning the DNA-nuclear skeleton interactions in isolated nuclear fractions were reviewed and discussed. Comparison of these different data clearly showed that the position of chromosomes observed in situ is in agreement with the results obtained from isolated nuclear fractions, indicating that all DNA molecules are bound to the peripheral nuclear skeleton. Moreover, the in situ position of the rDNA near the nuclear envelope can be correlated with the existence of a nucleolar skeleton connected to the peripheral nuclear skeleton. Taking into account the discrepant results regarding the actual existence of an internal nuclear skeleton, we attempted to analyze how the various nuclear skeletal structures described in the literature can be involved in both the distribution of chromosomes and in their chromatin organization. As many questions are still unanswered, we considered the modes of investigation that seem to be the most promising.

Association of DNA with the nuclear lamina in Ehrlich ascites tumor cells

We have studied in vitro binding of DNA to nuclear lamina structures isolated from Ehrlich ascites tumor cells. At low ionic strength in the presence of Mg++, they bind considerable amounts of mouse and bacterial DNA, forming complexes stable in 2 M NaCl. Single-stranded DNA and pulse-labeled DNA show higher binding efficiencies than native uniformly labeled DNA. When mixing occurs in 2 M NaCl, complex formation is inhibited. When nuclei are digested with DNAse I under conditions that favor chromatin condensation, DNA associated with matrices subsequently prepared from such nuclei is markedly enriched in satellite DNA. If digestion is carried out with DNAse II while nuclei are decondensed in EDTA, no enrichment in satellite DNA is observed. Preparations of purified, high-molecular weight, double-stranded DNA contain variable amounts of fast-sedimenting aggregates, which are insoluble in 2 M NaCl but are dispersed by DNA fragmentation or denaturation. These results point at some artifacts inherent in studies of DNA bound to residual nuclear structures in vivo and suggest conditions expected to avoid these artifacts. Further, using controlled digestion with DNAse II, we have studied the in vivo association of DNA with nuclear lamina isolated from Ehrlich ascites tumor cells. In the course of DNA fragmentation from above 50 kbp to about 20 kbp average size, the following events were observed. The DNA of high molecular weight (much longer than 50 kbp) behaved as if tightly bound to the nuclear lamina, as judged by sedimentation in sucrose and metrizamide density gradients, electron microscopy, and retention on glass fiber filters. As the size of DNA decreased, it was progressively detached from the nuclear lamina, and at about 20 kbp average length practically all DNA was released. The last 1-4% of DNA, although cosedimenting with the nuclear lamina in sucrose gradients, behaved as free DNA, banding at 1.14 g/cm3 in metrizamide density gradients and showing less than 4% retention on filters. At no stage of digestion did the DNA cosedimenting with nuclear lamina show changes in satellite DNA content relative to that of total DNA or enrichment in newly replicated DNA. It was shown, however, that digestion of nuclear lamina-DNA complex with EcoRI or Hae III led to the formation of DNA-protein aggregates, which banded at 1.35 g/cm3 in high salt containing metrizamide density gradients and which were strongly enriched in satellite DNA.(ABSTRACT TRUNCATED AT 400 WORDS)

Survival strategies of the yeast plasmid two-micron circle

The multicopy yeast plasmid 2-micron circle uses a number of strategies to insure its persistence in its host. The plasmid confers no selective phenotype to the cell in which it is resident. Nonetheless, the plasmid is lost at less than 1 per 10(5) cell divisions during continuous exponential growth. We have determined that the plasmid persists at least in part due to the ability of the plasmid to amplify its mean copy number when its cellular copy level is low and to distribute plasmid molecules equally between mother and daughter cells at mitosis. We have found that amplification of plasmid copy number occurs by a novel mechanism in which site-specific recombination induces a transient shift in the mode of replication from theta to rolling circle. Equitable partitioning of plasmid molecules requires plasmid-encoded proteins and a centromere-like segment on the plasmid. We have accumulated evidence consistent with a model of partitioning in which the partitioning proteins form a transnuclear structure that is responsible for distributing plasmid molecules throughout the nucleus prior to cell division. In this chapter we describe evidence supporting the existence and mode of action of these two plasmid strategies and discuss the extent to which these strategies may be a pervasive facet of the biology of eukaryotic extrachromosomal elements.

Lectin binding sites in Paramecium tetraurelia cells. II. Labeling analysis predominantly of non-secretory components

All the lectin-FITC conjugates tested (ConA, RCA II, WGA) bind to the surface of Paramecium cells. Yet only WGA yields a distinct fluorescent pattern; it contours the basis of cilia and in some cells it brilliantly stains a few neighbouring rows of the regular surface fields in the anterioventral region (a region known to contain extensive fields of linear aggregates of freeze-fracture particles and to be engaged in conjugation). Incubation in vivo with WGA-FITC resulted in the selective labeling of the cytopharyngeal region as well as of the cytoproct. On Lowicryl K4M sections, WGA-gold probes concomitantly labeled disk-shaped vesicles that are assumed in the literature to serve as shuttle vesicles between these two cell regions and, thus, to connect forming and defecating digesting vacuoles (stages DV I and DV IV). On K4M sections WGA-Au stains also most other components of the lysosomal system. Also on K4M sections RCA II-Au labeled the walls of bacteria contained in DV I and II type digesting vacuoles (but not lysosomes identified bona fide by their size and shape and by their frequent vicinity to or continuity with digesting vacuoles). The WGA data largely support previous conclusions on the possible functional connection of all these elements (DV I-IV, smaller lysosomes, disk-shaped vesicles etc.) of the lysosomal system in Paramecium, as proposed by Allen and his group on the basis of other lines of evidence. As shown in the accompanying paper, ConA-FITC stained ghosts (formed after massive trichocyst exocytosis) also abut into DV-like structures. The different results obtained with the three lectins tested reflect the complex sorting machinery contained in the elaborate lysosomal system of a Paramecium cell. In the cytosol, finally, there occurs a particularly intense staining with ConA-gold, applied to Lowicryl sections, that probably represents glycogen-like particles. The same procedure reveals some weak staining of secretory contents and of nuclear structures.

Nuclear substructure antigens. Monoclonal antibodies against components of nuclear matrix preparations

We describe two monoclonal antibodies, I-2 and I-14, which recognize, respectively, proteins of 36 and 40 kD. By immunofluorescence microscopy on chick embryo fibroblasts, both antigens were found to be located within a nuclear substructure which excludes nucleoli and part of the nucleoplasm; hence we refer to these antigens as nuclear substructure antigens. By immuno-electron microscopy on chick liver sections, the I-14 antigen was identified predominantly in clusters of interchromatin granules and in perichromatin fibrils. The two substructure antigens share a remarkable resistance to sequential extraction of nuclei with DNase I, RNase A, non-ionic detergent and high salt, indicating that they constitute part of an operationally defined residual nuclear matrix. Finally, both substructure antigens are virtually absent from the nuclei of adult erythrocytes. These properties suggest that substructure antigens may be involved in RNA transcription, processing or transport, possibly by contributing nucleoskeletal support.

Localization of ribosomal protein S1 in the granular component of the interphase nucleolus and its distribution during mitosis

Using antibodies to various nucleolar and ribosomal proteins, we define, by immunolocalization in situ, the distribution of nucleolar proteins in the different morphological nucleolar subcompartments. In the present study we describe the nucleolar localization of a specific ribosomal protein (S1) by immunofluorescence and immunoelectron microscopy using a monoclonal antibody (RS1-105). In immunoblotting experiments, this antibody reacts specifically with the largest and most acidic protein of the small ribosomal subunit (S1) and shows wide interspecies cross-reactivity from amphibia to man. Beside its localization in cytoplasmic ribosomes, this protein is found to be specifically localized in the granular component of the nucleolus and in distinct granular aggregates scattered over the nucleoplasm. This indicates that ribosomal protein S1, in contrast to reports on other ribosomal proteins, is not bound to nascent pre-rRNA transcripts but attaches to preribosomes at later stages of rRNA processing and maturation. This protein is not detected in the residual nucleolar structures of cells inactive in rRNA synthesis such as amphibian and avian erythrocytes. During mitosis, the nucleolar material containing ribosomal protein S1 undergoes a remarkable transition and shows a distribution distinct from that of several other nucleolar proteins. In prophase, the nucleolus disintegrates and protein S1 appears in numerous small granules scattered throughout the prophase nucleus. During metaphase and anaphase, a considerable amount of this protein is found in association with the surfaces of all chromosomes and finely dispersed in the cell plasm. In telophase, protein S1-containing material reaccumulates in granular particles in the nucleoplasm of the newly formed nuclei and, finally, in the re-forming nucleoli. These observations indicate that the nucleolus-derived particles containing ribosomal protein S1 are different from cytoplasmic ribosomes and, in the living cell, are selectively recollected after mitosis into the newly formed nuclei and translocated into a specific nucleolar subcompartment, i.e., the granular component. The nucleolar location of ribosomal protein S1 and its rearrangement during mitosis is discussed in relation to the distribution of other nucleolar proteins.

Three-dimensional distribution of Ag-NOR proteins in rat superior cervical ganglia nucleoli according to circadian rhythm

A three-dimensional reconstruction of the distribution of Ag-NOR proteins in nucleoli of sympathetic neurons of a rat killed during the dark period of its light-dark cycle was compared with previously reported analyses on the three-dimensional distribution of fibrillar centers, the high-resolution localization of these proteins, and the morphometric results. The domain occupied by these proteins appeared to far exceed that of the fibrillar centers and included the dense fibrillar RNP component. In the present material this component in turn provided partial bridging between the units consisting of the fibrillar centers plus their surrounding dense fibrillar component.

Nuclear development in locust fat body: the influence of juvenile hormone on inclusion bodies and the nuclear matrix

The hormonal induction of vitellogenesis in insects and in oviparous vertebrates are prime models of gene regulation in eukaryotes. In vertebrates the process is under estrogenic control and normally confined to females, although males can be artificially induced. In locust in contrast, juvenile hormone (JH) is central to fat body development in both males and females, yet the response is strongly sex limited not only for vitellogenin production but also in terms of total protein, DNA and RNA synthesis and nuclear ploidy levels. To differentiate further possible sex and/or JH related developmental aspects in locusts, large-scale nuclear events were examined during normal adult maturation and in animals treated with antiallatropins and JH analogs. Fat body nuclei undergo extensive restructuring during normal development in both sexes. This included progressive nuclear enlargement, accompanied by extensive proliferation of nuclear matrix components and elaboration of complex inclusion bodies (NB). The isolated protein matrix was unusually complex relative to similar structures from vertebrates and the NB were firmly anchored to it. Although matrix proteins were qualitatively similar to those from other sources, as assessed by SDS polyacrylamide gel electrophoresis, several major matrix polypeptides, including lamins A and B, and components greater than 150 kD, fluctuated quantitatively during development and in concert with nuclear enlargement. The number and morphology of the NB were unrelated to sex, but increased in direct proportion to absolute nuclear volumes. All changes were more pronounced in females, where higher ploidy levels, larger nuclei and correspondingly more internal matrix elements occurred. Suppression of JH production by precocene prevented all foregoing nuclear changes, but re-exposure to methoprene rapidly induced normal development. The results are compared to analogous nuclear changes in steroid responsive vertebrate tissues.

Isolation and characterization of the RNA2+, RNA4+, and RNA11+ genes of Saccharomyces cerevisiae

We used genetic complementation to isolate DNA fragments that encode the Saccharomyces cerevisiae genes RNA2+, RNA4+, and RNA11+ and to localize the genes on the cloned DNA fragments. RNA blot-hybridization analyses coupled with genetic analyses indicated the RNA2+ is coded by a 3.0-kilobase (kb) transcript, RNA4+ is coded by a 1.6-kb transcript, and RNA11+ is coded by a 1.3-kb or a 1.7-kb transcript or both; none of the cloned genes contains detectable introns. All three genes were transcribed into messages of very low abundance (approximately 20 times lower than a ribosomal protein message). DNA blot-hybridization revealed that all cloned genes are represented only once in the yeast chromosome. mRNA for RNA2+ and RNA4+ is produced in approximate proportion to gene dosage, whereas RNA11+ transcription appears to be not nearly so dependent on gene dosage. On a medium-copy plasmid (5 to 10 copies per cell), each cloned gene complemented mutations only in its own gene, indicating that each gene encodes a unique function. Genetic analysis by integrative transformation indicated that we cloned the RNA2+, RNA4+, and RNA11+ structural genes and not second-site suppressors.

Transcriptionally active chromatin

Eukaryotic chromatin has a dynamic, complex hierarchical structure. Active gene transcription takes place on only a small proportion of it at a time. While many workers have tried to characterize active chromatin, we are still far from understanding all the biochemical, morphological and compositional features that distinguish it from inactive nuclear material. Active genes are apparently packaged in an altered nucleosome structure and are associated with domains of chromatin that are less condensed or more open than inactive domains. Active genes are more sensitive to nuclease digestions and probably contain specific nonhistone proteins which may establish and/or maintain the active state. Variant or modified histones as well as altered configurations or modifications of the DNA itself may likewise be involved. Practically nothing is known about the mechanisms that control these nuclear characteristics. However, controlled accessibility to regions of chromatin and specific sequences of DNA may be one of the primary regulatory mechanisms by which higher cells establish potentially active chromatin domains. Another control mechanism may be compartmentalization of active chromatin to certain regions within the nucleus, perhaps to the nuclear matrix. Topological constraints and DNA supercoiling may influence the active regions of chromatin and be involved in eukaryotic genomic functions. Further, the chromatin structure of various DNA regulatory sequences, such as promoters, terminators and enhancers, appears to partially regulate transcriptional activity.

Drug-induced dispersal of transcribed rRNA genes and transcriptional products: immunolocalization and silver staining of different nucleolar components in rat cells treated with 5,6-dichloro-beta-D-ribofuranosylbenzimidazole

Upon incubation of cultured rat cells with the adenosine analogue 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), nucleoli reversibly dissociate into their substructures, disperse throughout the nuclear interior, and form nucleolar "necklaces". We have used this experimental system, which does not inhibit transcription of the rRNA genes, to study by immunocytochemistry the distribution of active rRNA genes and their transcriptional products during nucleolar dispersal and recovery to normal morphology. Antibodies to RNA polymerase I allow detection of template-engaged polymerase, and monoclonal antibodies to a ribosomal protein (S1) of the small ribosomal subunit permit localization of nucleolar preribosomal particles. The results show that, under the action of DRB transcribed rRNA, genes spread throughout the nucleoplasm and finally appear in the form of several rows, each containing several (up to 30) granules positive for RNA polymerase I and argyrophilic proteins. Nucleolar material containing preribosomal particles also appears in granular structures spread over the nucleoplasm but its distribution is distinct from that of rRNA gene-containing granules. We conclude that, although transcriptional units and preribosomal particles are both redistributed in response to DRB, these entities retain their individuality as functionally defined subunits. We further propose that each RNA polymerase-positive granular unit represents a single transcription unit and that each continuous array of granules ("string of nucleolar beads") reflects the linear distribution of rRNA genes along a nucleolar organizer region. Based on the total number of polymerase I-positive granules we estimate that a minimum of 60 rRNA genes are active during interphase of DRB-treated rat cells.